= Solution
For steady currents in vacuum, <Ampère's circuital law> is
$$
\oint_C B\cdot d\ell=\mu_0I_{\mathrm{enc}}.
$$
Here $C$ is an oriented closed <curve>, $d\ell$ its tangent line element, and $I_{\mathrm{enc}}$ the signed <electric current> through a spanning <surface> oriented by the <right-hand rule>. A circular contour about a straight infinite wire has constant tangential <magnetic field>, so $B(r)=\mu_0I/(2\pi r)$. The <Lorentz force> on a length $L$ of the other wire is $IL\hat\ell\times B$. Therefore
$$
\boxed{\frac FL=\frac{\mu_0I^2}{2\pi r},\qquad\text{the force is attractive}.}
$$
The <right-hand rule> makes the force on each parallel, same-direction current point toward the other wire.
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